The invention relates to the field of electronic reproduction technology and is directed to a method and to an apparatus for the correction of positioning errors of a light ray in a flying spot laser scanner deflected Point-by-point and line-by-line by a deflection system comprising at least one mirror surface, particularly by a multi-face rotating mirror.
Flying spot laser scanners are employed in electronic reproduction technology as input scanners for point-by-point and line-by-line scanning of originals or as an output scanner for recording information. In an input scanner, a light ray is conducted over an original point-by-point and line-by-line with a deflection system and the scan light allowed to pass by or reflected from the original is converted into an image signal in an opto-electronic transducer. In an output scanner (recorder), a light ray that is intensity-modulated by an image signal is conducted across a recording medium point-by-point and line-by-line with a deflection system. The original or recording medium, referred to below as a surface, thereby move continuously or in steps perpendicularly to the line direction of the deflected light ray.
In order to achieve a high reproduction quality, a high deflection precision of the light ray must be maintained during original scanning and recording. Among those things required are that the spacings of the lines are identical and that the line starts and line ends on the surface lie on lines proceeding exactly perpendicularly to the line direction.
As a result of optical-geometrical aberrations of a deflection system, positioning errors of the deflected light ray arise on the surface, the precision being deteriorated as a result thereof.
The cause of such optical-geometrical errors, for example, are tolerances in the manufacture of the deflection system and deficiencies in the mechanical structure.
Mirror faces that are not arranged parallel to the rotational axis of the deflection system and an unstable position of the rotational axis cause positioning errors of the light ray perpendicularly vis-a-vis the line direction and, thus, cause unequal line spacings on the surface. Different angles between abutting mirror faces cause positioning errors in line direction and, thus, unequal line starts and line ends on the surface. Further positioning errors can arise due to uneven mirror faces. These positioning errors that are expressed by a non-uniform angular speed of the deflection system and, thus, by distortions within the lines are especially disturbing when a high precision or, respectively, reproduction quality is demanded.
EP-B-0054170 already discloses a means for the correction of a light ray in a flying spot laser scanner deflected by a deflection system in the form of a multi-face rotating mirror. In the known means, correction deflectors for the light ray are arranged between the laser generator for generating the light ray and the deflection system in order to compensate the positioning errors of the light ray caused by the deflection system. The positioning errors of the light ray are measured in a measurement phase preceding the actual operation. Correction values for driving the correction deflectors are then generated and the correction values are modified while constantly checking the positioning errors until the positioning errors are compensated by the correction deflectors. The correction values required for compensating the positioning errors are stored and are output for ongoing compensation of positioning errors during the actual operation synchronized with the rotational movement of the deflection system. The correction deflectors are piezoelectrically driven tilting mirrors or acousto-optical deflectors.
The known apparatus has the disadvantage that additional correction deflectors are required therein for compensating the positional errors and that the piezoelectrically driven tilting mirrors or acousto-optical deflectors have intrinsic errors that do not allow an optimum correction. For example, a piezoelectrically driven tilting mirror has a nonlinear characteristic affected with hysteresis and, due to its relatively great mass, only achieves a limited deflection speed. Added thereto is that the temperature dependency and the aging of the piezoceramic require a more frequent balancing of the apparatus.
Given an acousto-optical deflector, the optical efficiency unfavorably varies with the deflection angle and an acousto-optical deflector only allows a small ray diameter, as a result whereof its area of utilization is restricted. Moreover, a relatively involved frequency control is required for the operation of such an acousto-optical deflector.